In filling and holding pressure drop section, cavity pressure increases with time. After filling cavity, pressure will remain in a relatively static state to supplement insufficient amount of glue due to shrinkage. In addition, this pressure can prevent phenomenon of colloid backflow caused by reduction of injection. This is pressure holding stage. After pressure holding is completed, cavity pressure gradually decreases, and can theoretically drop to zero over time, but it is not actually zero. Therefore, after demolding, internal stress of product is stored, so some products need to be post-processed to remove residual stress.
Tag: back pressure
Property of friction between macromolecules when molten plastic flows is called plastic viscosity. Coefficient of this viscosity is called viscosity, so viscosity is a reflection of fluidity of molten plastic. The greater viscosity, the stronger viscosity of melt, the worse fluidity and the more difficult processing. In industrial applications, comparing fluidity of a plastic is not based on its viscosity value, but on its melt flow index (called MFI): so-called MFI means that at a certain melting temperature, melt is subjected to a rated pressure, weight of melt passing through standard die per unit time (usually 10 minutes). It is expressed in g/10min, such as injection molding grade PP material, grade is different, MFI value can vary from 2.5 to 30. Viscosity of plastic is not static. Change of characteristics of plastic itself, influence of external temperature, pressure and other conditions can all contribute to change of viscosity.
Debugging of injection molding machine refers to continuous adjustment of various parameters of injection machine for specific mold until qualified plastic parts are produced. Various parameters of plastic injection machine can be roughly classified as follows:
For hygroscopic materials (such as nylon), high water content of resin will also cause screw to slip. Improper drying will significantly reduce viscosity of material and cause air bagging in screw, thus reducing conveying capacity of screw. In this regard, a humidity analyzer can be used to determine moisture content of dried material before processing.
Melt usually flows from nozzle into sprue, but in some molds, nozzle is part of mold because it extends to bottom of mold. There are also two main types of nozzles: open nozzles and closed nozzles. In injection molding production, open nozzles should be used more because they are less expensive and less likely to hold up. Even low viscosity melts can be used with this nozzle if injection molding machine is equipped with a depressurization device.
Valve cover of forklift truck is shown in Figure 1. Features that affect design of mold structure are: â‘ 4 inverted buttons C1~C4 formed on cavity side; â‘¡ 3 holes H1~H3 on cavity side, of which hole H1 is an inclined hole for bidirectional core pulling and demoulding; â‘¢ 5 inverted buttons K1~K5 formed on core side; â‘£ 5 threaded posts B1~B5 formed on core side.
1. Briefly describe working state of check valve in process of plasticization, injection and pressure holding; Plasticization: On Injection molding: off Holding pressure: off
Before molding, plastic must be fully dried. After material containing moisture enters mold cavity, silver ribbon-like flaws will appear on the surface of workpiece, and even hydrolysis will occur at high temperatures, resulting in material deterioration. Therefore, material must be pretreated before molding process, so that material can maintain appropriate moisture.
Before injection molding, plastic must be fully dried. After material containing moisture enters mold cavity, it will cause silver tape-like flaws on the surface of part, and even hydrolysis at high temperatures, resulting in deterioration of material. Therefore, material must be pretreated before molding process, so that material can maintain proper moisture.
Melt glue usually flows from nozzle into nozzle, but in some molds, nozzle is part of mold because it extends to bottom of mold. There are also two main types of nozzles: open nozzles and closed nozzles. In injection molding, open nozzles should be used more because they are cheaper and less likely to stay. If injection molding machine is equipped with a pressure-removing device, this nozzle can be used even for low-viscosity melts. Sometimes a closed nozzle must be used. This nozzle acts as a stop valve to block plastic in injection cylinder. Make sure that nozzle is correctly connected to nozzle sleeve. Top hole is slightly smaller than nozzle sleeve. This makes nozzle easier to withdraw from mold. Hole of nozzle sleeve is 1mm larger than that of nozzle, that is, radius of nozzle is 0.5mm smaller than radius of nozzle sleeve.
Gud Mould 